1. Field of the Invention
The present invention relates to amplitude modulation, and in particular to amplitude modulation of phase modulated signals.
2. State of the Art
Amplitude modulation (AM) is the technique of superimposing an information signal onto a carrier signal while maintaining the same frequency and phase of the original carrier signal for transmission on a communication medium. One technique of amplitude modulation is performed by utilizing linearly phase modulated signal(s) to generate a corresponding amplitude modulation signal. According to this technique, amplitude modulation is performed by initially splitting a single RF carrier signal into two separate channel signals having a fixed angle (or phase) separation less than 180 degrees apart. The two channel signals are then phase modulated by out-of-phase modulation signals. Next, the phase modulated signals are amplified in their separate channels by non-linear power amplifiers to ensure sufficient amplification for transmitting the signal. The amplified signals are then combined to produce a single AM signal. Amplitude modulation in this system is realized by the combination/summation of the relative phase modulated channel signals which when combined create an amplitude modulation effect due to amplitude addition/subtraction of the two channel signals. In the past, this type of modulation has also been referred to as Chireix Outphasing Modulation, and LINC (linear amplification using non-linear components).
FIG. 1A shows a basic design of this type of amplitude modulation system wherein a low level RF signal 10A Cos(xcfx89ct+xcfx86(t)), where ∞xe2x89xa6xcfx86(t)xe2x89xa6∞, is coupled to signal processing block 10. Signal processing block 10 splits RF signal 10A into two separate signals out-of-phase from each other by a fixed amount. In addition, each of the two signals are phase-modulated with opposite phase information or modulation signals 10B (i.e., phase signals that are 180 degrees out-of-phase from each other) to generate two separate channel phase modulated RF signals 10C and 10D where signals 10C and 10D are out of phase by some amount xcex8(t) less than 180 degrees:
Signal 10C=A1 Cos(xcfx89ct+xcfx86(t)+xcex8(t))
Signal 10D=A1 Cos(xcfx89ct+xcfx86(t)xe2x88x92xcex8(t))
Each of signals 10C and 10D are then amplified by identical constant amplitude non-linear power amplifiers 11 and 12 to generate signals 11A and 12A:             Signal      ⁢              xe2x80x83            ⁢      11      ⁢      A        =                            A          M                2            ⁢              Cos        ⁡                  (                                                    ω                c                            ⁢              t                        +                          φ              ⁡                              (                t                )                                      +                          θ              ⁡                              (                t                )                                              )                                Signal      ⁢              xe2x80x83            ⁢      11      ⁢      B        =                            A          M                2            ⁢              Cos        ⁡                  (                                                    ω                c                            ⁢              t                        +                          φ              ⁡                              (                t                )                                      -                          θ              ⁡                              (                t                )                                              )                    
which are then combined by the summation block 13 to generated the AM signal 13A.
The advantage/motivation of performing modulation in this manner is to improve the energy efficiency of the power amplifiers. By converting the amplitude-modulated signal into a pair of phase-modulated signals, the amplifiers no longer have to follow the amplitude modulation. With the amplifier input signal now of constant envelope, the amplifier can be operated in a non-linear manner, which is a well-known requirement for high efficiency.
For example, FIG. 1B illustrates a direct transition along a line 14 in a 16-point QAM (quadrature amplitude modulated) signal. FIG. 1C shows the phase and FIG. 1D shows the magnitude of the desired signal 14 during the transition. FIGS. 1C and 1D also show the phases of the fixed-magnitude channel signals (15 and 16) of this technique. In these figures, transition position refers to the ordinate (xe2x80x9cyxe2x80x9d axis) of a point on line 14 in FIG. 1B.
However, the disadvantage of this type of modulation scheme is that two equal sized power amplifiers are required to amplify each of the channel signals. The major problem with using this type of amplifier is that they tend to be relatively large components having high power requirements resulting in a substantial increase in system size and power consumption.
Also, both of power amplifiers 11 and 12 together continuously generate the transmit peak envelope power (PEP). As a result, as the output signal magnitude falls below the PEP, the difference power must be absorbed by the output combiner (block 13) as shown in FIG. 1F. In addition, to obtain the desired amplitude modulation, the phase modulation rates of the components can be significantly higher than the desired phase modulation rate, particularly at low output magnitudes.
Furthermore, some transistors, notably bipolar junction transistors (BJT), when used as power amplifiers in this type of system, have distortions in their operating characteristics at low signal magnitudes. Thus, generating low level output signals using efficient techniques such as those taught in U.S. patent application Ser. No. 09/297,097 entitled xe2x80x9cHigh Efficiency Amplifier Output Level and Burst Control,xe2x80x9d incorporated herein by reference, could incur undesired distortions when using these types of transistors.
What would be desirable is to perform this type of modulation technique at reduced complexity and increased efficiency while substantially reducing the size and power of the modulation system.
It is an objective of this invention to provide for the generation of amplitude-varying passband signals using only amplifiers which are either non-linear or extremely non-linear (switch-mode).
It is an additional objective that the complexity of the dual-phase-modulated signal synthesis block of the Chireix or LINC methods be reduced.
It is another objective that this modulator implement the carrier phase reversals of an amplitude-modulated-only signal, without the need of phase modulator circuitry.
It is a further objective that the overall energy efficiency of this signal generator be high.
A still further objective is for this signal generator to not require more modulation bandwidth than that of the desired signal.
A still further objective is for this signal generator to not require large amounts of heat to be absorbed by the combiner block when the output signal is at low amplitude.
Furthermore, it is an objective of the present invention to assure that efficient non-linear amplifiers used in the modulation system always operate above the onset of any distortion occurring on low-level signals, while maintaining full control on the output signal for vanishingly small output levels.
In order to achieve these objectives, a system and method of dual channel amplitude modulation of a phase-modulated signal is described.
One embodiment of the system includes means for producing first and second phase modulated signals that are 180 degrees out of phase from each other. The system further includes a first non-linear amplifier and a second non-linear amplifier having an amplification factor much greater than the first non-linear amplifier. The first non-linear amplifier amplifies the first signal with constant amplification to generate an amplified first signal. The second non-linear amplifier amplifies the second phase modulated signal by performing varying amplification using an information signal to generate an amplitude modulated and amplified second signal. A signal combiner sums the output signals from the two non-linear amplifiers. Since the signals generated by the first and second non-linear amplifiers are 180 degrees out of phase from each other and since the first non-linear amplifier has an amplification factor that is significantly less than the amplification factor of the second non-linear amplifier, the output signal from the signal combiner is the larger modulated signal minus the smaller opposing signal. The resulting effect is that the signal generated by the first amplifier cancels out distortion at the lower amplitude levels in the second modulated signal thereby generating an amplitude modulated signal having reduced lower level distortion.
One embodiment of the method of amplitude modulating a phase modulated signal includes the steps of producing first and second phase modulated signals that are 180 degrees out-of-phase from each other. The first phase modulated signal is then amplified by a first smaller amplification factor using constant amplification to generate an amplified first signal. The second phase modulated signal is amplitude modulated and amplified using variable amplification by a second amplification factor to generate an amplitude modulated and amplified second signal. Next, the amplified first signal and the amplitude modulated and amplified second signal are summed to generate an amplitude modulated signal of the original phase modulated signal having reduced distortion at lower amplitude levels.